Some varieties of flexible electronic printed circuit backings are comprised of impregnated non-woven fibrous web materials laminated to an electrically conductive metal foil, which after suitable selective etching provides the electrical circuitry.
One such non-woven fibrous web material having particular utility herein is disclosed in commonly assigned and copending application Ser. No. 577,444, now Pat. No. 4,035,694. The web disclosed therein comprises a fiber blend of high tensile strength and heat-resistant discontinuous fibers, the fibers being compacted and held together with a matrix of film-forming polymeric binder material. The web has an extreme degree of dimensional stability during the manufacturing and subsequent processing operations of the printed circuitry.
An adhesive system is typically applied to the compacted web prior to lamination and bonding thereto of the conductive metal foil.
The adhesives used typically are thermosettable resin systems which are flexible, have high insulation resistance, are not adversely affected by molten solder temperature conditions, i.e., about 500.degree. F., etc. Typically, such systems are latent curing to provide adequate shelf life when in a semi-cured, or B-stage condition. Furthermore, the adhesive system should afford high peel adhesion to the electrically conductive metallic foil layer.
One further criteria necessary for the dielectric sheet material is that of flame retardancy. It has been observed that when flame retardancy is desired or necessary, it has been almost impossible to achieve the combination of foregoing characteristics in addition thereto. Adhesive systems capable of providing for requisite electrical properties typically cure too fast to be suitably processable, and systems with the required latency do not meet the ultimate performance properties necessary for the sheet material. Furthermore, flame retardancy attained via bromination of the adhesive system, i.e., by simply adding a highly brominated compound thereto, has been found to considerably reduce the flexibility of the dielectric material.
It has now been found that a particularly defined epoxy/polyester adhesive system is capable of attaining the aforementioned characteristics in addition to flame retardancy.